1. Field of the Invention
Aerosol containers are usually pressurized to approximately 70 psi at room temperature. This pressurization enables the contents to be expelled via a valve controlled by the user. Most aerosol containers are of three-piece construction consisting of a seamed side wall, an outwardly domed top wall and an inwardly domed bottom wall. The top and bottom walls are fastened to the cylindrical side wall by a mechanically formed double seam, with a sealing compound incorporated into the seam. The valve assembly is mechanically attached to the top wall after the container is filled. Two-piece containers are also used, wherein one end and the side wall are made in one piece by forming.
Pressurization of the container is achieved by adding a high vapor pressure fluid, or propellant, to the product during the final step of the filling process. The propellant maintains an internal pressure within the container from full to empty. In the past, chlorinated fluorocarbons were used as aerosol propellants, but their use has been discontinued in aerosol packaging due to environmental consideration. The propellants currently used typically are butane, isobutane, propane or a mixture of them. All of these propellants are highly flammable and present a fire hazard. They are similar to chlorinated fluorocarbons in pressure-temperature relationship. This has allowed them to be substituted for chlorinated fluorocarbons in aerosol containers without change. Non-flammable, environmentally safe propellants are available, but are not currently economical alternatives. The vapor pressure of these propellants is significantly higher than the pressures of current or past propellants. To utilize these propellants in an aerosol container, requires changes to the container to allow safe containment of the contents at the increased internal pressure. To date, changes to the container allowing increased pressure containment capacity have not been economically successful.
Changing to a non-flammable, environmentally safe propellant is desirable. Achieving this will require an improved aerosol container capable of increased internal pressure containment capacity due to the increased pressures associated with these propellants. Solely achieving higher pressure containment capacity in an aerosol container will increase the temperature and pressure at which the container bursts, but will increase the potential danger from bursting. The danger arises from the increased stored energy due to the higher pressure storage of product and propellant. This increased stored energy will be released as kinetic energy if the container bursts.
Therefore, it is important that a higher pressure aerosol container vent rather than burst. The controlled release of the aerosol container contents, prior to a pressurization that could cause catastrophic or explosive rupturing of the container, will avoid the dangers due the bursting of an aerosol container. In order to provide a balanced approach to a safer aerosol container, it must incorporate higher internal pressure capacity features allowing the use of non-flammable environmentally safe propellants, while also including a pressure release system for controlled release of the container pressure during over pressurization conditions.
2. The Prior Art
The prior art has approached the improvements for increases in internal pressure capacity of the aerosol containers and the formulation of a controlled pressure release system separately. Past efforts to increase the pressure containment capacity of aerosol containers has primarily centered around material strength. The thickness and/or strength of the material were increased to provide a stronger container with inherently more pressure containment capacity. The containers resulting from these efforts are not economical for production and therefore are not viable. Several of the more typical high-pressure container designs have not been able to become economically viable due to the cost of the materials used in each container and manufacture costs. However, increasing the internal pressure of current production containers causes failures in the mechanical attachment of the ends to the body, not in the material from which the container is constructed.
Past efforts to provide for pressure release in aerosol containers have centered around mechanical devices and/or introduction of artificial weakness to the container. The devices would open a valve or the like in the container when the internal pressure reached a specific level. U.S. Pat. No. 3,714,965 to Bentley disclose a pressure activated valve on the container. Pressure activated valves integrated into the valve cup assembly are disclosed in U.S. Pat. Nos. 3,722,759 to Rodden and 3,866,804 to Stevens. U.S. Pat. No. 3,912,130 to Pelton discloses vents in the double seam of the dome, which vents open when the dome buckles due to over pressurization. These devices however have been plagued by such problems as high acquisition costs, manufacturing difficulties and unacceptable performance reliability. As a result, these devices have not been widely incorporated into commercial production aerosol containers in any form.
The introduction of artificial weakness into aerosol containers have recently had limited commercial production application. U.S. Pat. Nos. 3,850,339 to Kinkel, 4,513,874 to Mulawski and 4,588,101 to Ruegg disclose devices of this nature. These weaknesses can be broadly characterized as scores in the metal that are intended to locally fracture the material when a specific pressure range is reached or a specific over pressurization event occurs, such as to outwardly buckle the dome. These pressure release mechanisms are highly dependent on the manufacturing processes and control introducing the scoring to the metal. For the weakened area to fracture at the proper pressure, the tolerances of the manufacturing process must closely be controlled and the material must meet very precise specifications with consistency.